EARLY CAMBRIAN STRATIGRAPHY. 
119 
fauna on Dearborn river, Montana. Acrothele panderi 1 
and Wimanella simplex 2 are confined to the Albertella fauna. 
Olenopsis agnesensis 3 and 0 . americanus 4 are the coordinate 
representatives in correctable beds of a genus which has not 
hitherto been identified in North America but which Mr. 
Walcott 5 refers to the Lower Cambrian on the basis of 
the ‘‘Lower Cambrian” age of the Albertella fauna and 
describes as “a form intermediate between Holmia (restricted) 
and Paradoxides 6 or * * * descendant from the Holmia 
type of the Mesonacidae.” Albertella bosworthi and A. Helena 
are described by Mr. Walcott 7 as representing most interesting 
types of the Paradoxidse which “should first be compared with 
the genus Zacanthoides which, in the British Columbia section, 
is first met with in strata 2,000 feet above the beds in which 
Albertella occurs.” Bathyuriscus productus is the represen- 
tative of a genus typical of the Middle Cambrian in the 
Cordilleran region. It occurs in the Spence and Stephen 
formations and is one of the two species mentioned by Mr. 
Walcott 8 as characteristic of the upper or Middle Cambrian 
portion of the Pioche formation. 
With the single exception of Micromitra ( Tphidella ) pannula, 
therefore, none of the twelve described species of which this 
fauna is composed are known to occur in rocks older than 
the horizon under discussion. Moreover, two of the types 
(Albei'tella and Olenopsis ) seem to be derived from the old and 
to be prophetic of the new, and the remainder are either confined 
to the Albertella fauna or are typically to be referred to the 
Middle Cambrian. 
The problem before us does not seem sufficiently complicated 
to require a discussion of the principles governing the delimita- 
tion of stratigraphic units, since, by its attitude toward both 
of the elementary lithologic and organic propositions (a) that 
nValcott: Mon. U. S. Geol. Survey, vol. 51, 1912, p. 652. 
2 Idem, p. 748. 
"Walcott: Smithsonian Misc. Coll., vol. 57, No. 8, 1912, p. 242. 
"Idem, p. 243. 
"Idem, p. 239. 
"The writer does not share this view of the phyletic relations of Olenopsis, but 
believes with Mr. Walcott that it is probably descendant from the Mesonacidse. 
7 Smithsonian Misc. Coll., vol. 53, No. 2, 1908, p. 18. 
“Bull. U. S. Geol. Survey, No. 30, 1886, p. 39. 
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